End milling machines for certain parts that must be precisely oriented and located in relation to pre-existing features on the workpiece, have previously required hand loading to reliably position the workpiece in a holding chuck. This is especially true of scroll parts. As a result, total production time is slowed and inconsistent due to the hand loading operation.
This invention provides the necessary scroll part positioning combined with self-loading and unloading of the workpiece, utilizing the same horizontal and vertical machine slides that are used for machining moves.
The facilities for the load-unload operation include:
(A) conveyor for conveying multiple pallets carrying oriented workpieces; PA1 (B) A pallet escapement mechanism to pre-position a pallet in the loading station; PA1 (C) Precise positioning of the pallet in the loading station by downward engagement of two chisel pointed splines into two receiving grooves in the pallet; PA1 (D) Firmly seating the workpiece against rest buttons in the part chuck by continuing downward movement against a compliant conveyor bed after the initial rest button contact; PA1 (E) Part jaw clamping of the workpiece with detection of loading errors or improperly machined workpieces; PA1 (F) Further detection of improper projection of the clamped part, by passage of the workpiece across an interference sensing wall, to sense possible workpiece misposition and inhibit further machine motion; and PA1 (G) Finish machined workpieces are returned to their original pallet by the machine slides. The following pallet is then indexed ahead and the next machine cycle begins.
Previously, the scroll workpieces have been chucked in the horizontal attitude, even though they are conveyed to the machine in the vertical attitude. This required a separate loading motion to rotate the workpiece 90.degree. for loading. This invention chucks and mills the scroll workpiece in the same attitude in which it is conveyed.
To precisely mill workpiece size requires an accurate pre-semi-finish operation to remove rough stock and present workpieces (with a uniform amount of stock for removal) to a subsequent finish mill operation. Previously, semi-finish operations were performed in different machines and in different chucks than those for the finish mill operation. Lack of repeatability of position in the first and second chucking causes variation in the stock that must be removed in the finish mill operation. Also, when workpieces are semi-finished in multiple semi-finish machines, a variation in the semi-finish dimensions results between machines. In addition, within just a single semi-finish machine, there are still dimensioned trend changes due to various factors such as tool wear, tool exchange, thermal growth, etc. If these workpieces do not follow in sequence to the finish operation, the finish operation will see abrupt changes in the amount of finish stock to be removed, resulting in inaccuracy.
This invention semi-finish mills and finish mills the workpiece in the same chucking operation. Also workpieces are finish milled in the same sequence in which they were semi-finished. In this way, there is no loss of workpiece position due to rechucking, and the finish operation is not presented with abrupt workpiece size changes due to being processed out of sequence.
The assembly of scroll components into a scroll compressor requires precise location of the relative parts. Scroll features are machined to provide the necessary alignment. Previously, these alignment features were milled in a separate machine. Lack of repeatability of position in the first and second chucking caused machining errors and, subsequently, misalignment of the scroll components.
In accordance with the invention, the alignment features are milled in precise alignment with the scroll by completing the alignment milling without rechucking after the scroll is milled.
In milling, chips must be removed from the cutting area to prevent recutting the chips, which can cause poor finish and inaccuracy. Previously, the workpieces have been milled in the horizontal attitude with the chips tending to lay in the machining cavity of the workpiece.
This invention, with its inverted scroll attitude, provides gravity assistance in removing chips from the machining cavity.
Semi-finishing of the face of the scroll wall has previously been milled using the end of the same end mill as used for milling the sides of the walls. The long path along the scroll requires considerable cycle time and the tool wear which occurs in the long path shortens the usable life of the end mill.
The invention provides an additional machining station, equipped with a large, long-life face mill, to mill the full face of the scroll in one quick short pass and remove wear effect from the semi-finish end mill.
Milling operations generate a heavy coolant mist. Previously, the opening of loading doors has allowed some of this coolant mist to escape and become a hazard to both the operator's health and the machine mechanism.
This invention provides a virtually closed machining area, thus preventing escape of the coolant mist.
In milling of the walls of a scroll workpiece, the side pressure on the end mill causes the end mill to deflect in a cantilever manner resulting in taper of the milled scroll walls. This invention incorporates the invention of copending application of Garnett and Watson (owned by a common assignee), Ser. No. 08/443,011, filed May 17, 1995, to correct wall taper due to end mill side deflection, and the disclosure of which is incorporated herein by reference.
Previously, dimensioned accuracy of the milled workpiece had to be determined in a separate gaging machine, followed by human assessment of how much tool offsets needed to be corrected before milling subsequent parts. These offsets have had to be manually entered in the control.
A machine constructed in accordance with this invention automatically corrects for all inaccuracy, of (A) variations in wall taper, (B) wall position, and (C) variation from the theoretical involute path of the scroll. A gaging station is provided to gage an occasional workpiece and feed back corrections to the tool offset program for subsequently milled workpieces.
Passage of the scroll part over a series of multiple stations requires a long stroke and, to reduce the cycle time needed for the long stroke, a high velocity of travel is needed. To provide precision position control of the slide in critical areas wherein a ball screw is utilized, the ball screw nut must be preloaded resulting in a shortened life of the ball screw and ball nut arrangement. In the workpiece loading and face milling areas, the position control is not as critical and the preload may be reduced to extend the ball screw life. The machine of this invention incorporates variable preloading of the ball screw nut.
To assure precise depth milling of the workpiece, the machine design should direct the milling force vector as nearly as possible to the supporting ways.
The machine of this invention has the rotary axis built into the vertical slide to provide a minimum of rotary axis overhang from the supporting slides for rigid vertical positioning.
End mills used in the milling operation are a perishable item and must be frequently replaced. For convenience, the end mills should be positioned in full view of the machine operator at a convenient height and within easy reach. This invention provides upward facing mills and short reach access.
Spindles used for the milling tools in scroll machining operations are necessarily high rpm spindles and, as such, the bearings have a relatively short life. To reduce machine down time during servicing, this invention provides easy vertical removal and replacement of the spindles.
Since spindles and spindle arrangement requirements vary between scroll manufacturers, this invention provides a spindle containment tub or receptacle for all of the spindles, which can readily be exchanged with an alternate arrangement without affect on the remainder of the machine structure.
Some scroll designs include keyway slots which must be precisely positioned in relation to the scroll contour. This invention provides an additional servo controlled axis to accomplish the keyway machining of keyway slots which are offset from the centerline of the scroll part. Since the keyway is machined in the same chucking as used for the scroll machining, errors caused by rechucking are eliminated. This feature is also useful in machining some scroll features.
In operation of a machine tool, heat is introduced into the machine structure. This, along with variation in the ambient temperature of the factory, causes thermal distortion in the machines and inaccuracy in the machining. Also, forces generated by machining forces within a machine cause elastic deformation and inaccurate machining. To minimize these two effects, it is desirable to closely couple the workpiece holding chuck to the milling tool through a structural path which is as short as possible. This invention provides the desired closely coupled arrangement as compared to other arrangements in which the coupling is remote, requiring extensive auxiliary cooling of the machine structure and providing less resistance to distortion due to machining stresses.
The machine of this invention is self-contained with all facilities mounted on a single pair of skid rails for easy placement on the factory floor and therefore reduces the size of the required and expensive floor space, as compared to a machine with separate electrical control, separate spindle coolant facilities, separate hydraulic supply unit and others.
One object of this invention is to provide a milling machine having the foregoing features and capabilities.
Another object is to provide a milling machine which is rugged and durable in use and is capable of rapidly and accurately semi-finishing the work and then finishing the work to final form, all in a single machine and in a single chucking operation.
These and other objects, features and advantages of the invention will become more apparent as the following description proceeds, especially when considered with the accompanying drawings.